In the conventional methods used for manufacturing spark plugs, a center electrode assembly is mounted in an insulator bore and a ground electrode is welded to a shell prior to mounting the insulator in the shell. The insulator has a stepped bore in which the center electrode assembly is mounted. The center electrode assembly includes a center electrode having an enlarged diameter head or shoulder which is seated on the insulator bore step and a tip which projects from an insulator firing tip or nose for forming a spark gap with the ground electrode on the shell. The center electrode may include an extendion above the shoulder. A powdered sealing material, such as talc, is tamped in the annular space between the wire, the center electrode shoulder and the insulator to retain the center electrode and to form a seal. In one conventional type of spark plug, a terminal is threaded and cemented into the upper end of the insulator bore in contact with the center electrode wire to complete the center electrode assembly. In another type of spark plug, a spring and a resistor or an ignition noise suppression element are positioned in the insulator bore and a terminal is threaded and cemented into the upper bore end to complete the center electrode assembly. In that situation the spring is compressed to maintain series electrical continuity between the terminal, the resistor and the center electrode wire. In still another type of spark plug, an electrically conductive or semi-conductive powder is tamped under high pressure in the insulator bore above the head or shoulder on the center electrode. The tamped powder retains the center electrode, forms a seal and maintains electrical continuity in the center electrode assembly. When the tamped material has semi-conducting properties, it also may function as an ignition noise suppressor. A spring is inserted into the insulator bore and a terminal is threaded and cemented into the upper end of the bore to complete the center electrode assembly. Again, the spring maintains series electrical continuity between the terminal and the tamped powder.
Various methods are used for assembling a spark plug insulator and shell. In one common assembly method, a gasket is positioned on an internal shoulder or step in the shell. The insulator then is positioned in the shell so that a shoulder or flange on the insulator seats on the gasket. A powder sealing material is tamped under high pressure into the annular space between the insulator, the insulator shoulder and the shell to firmly hold the insulator in place and to form a seal. Finally, the upper edge of the shell is rolled inwardly to retain the powder.
In a modified method for assemblying the insulator and the shell known as the "hot press" method, the insulator is placed in the shell with a radially extending flange or shoulder seated on a step or shoulder in the shell. A gasket then may be positioned on top of the insulator flange and the upper edge of the shell is rolled inwardly to retain the insulator in the shell. A high electric current is passed longitudinally through the shell to heat a thin walled section. Or, the thin walled shell section may be inductively heated. While the thin walled section is hot, the shell is pressed and axially collapsed at the thin walled section. The pressure is maintained while the shell cools. During this process, the shell shoulder and/or gasket is deformed slightly by the insulator shoulder to form a seal between the shell and the insulator.
In a "cold press" method of assemblying a spark plug insulator in a shell, the insulator is positioned in the shell with a radial flange or shoulder on the insulator seated on a shell shoulder, or on a gasket which is positioned on the shell shoulder, and a powdered sealing material is tamped in the annular space above the insulator flange between the insulator and the shell. The upper end of the shell then is rolled inwardly over the upper end of the tamped powder and the shell is axially collapsed at a thin walled section by applying a high axial pressure to the shell. The pressure slightly deforms the shell shoulder or the gasket to form a seal between the insulator and the shell. The compressed resilient powder above the insulator shoulder holds the insulator shoulder firmly against the shell shoulder and also forms a seal.
When the insulator and shell are assembled by any of the above methods, a tip of the center electrode assembly projects from the insulator for forming one side of a spark gap. Because of normal manufacturing tolerance variations in manufacturing the center electrode, the insulator and the shell and in assemblying these components, there may be considerable variation in the location of the center electrode tip projecting from the insulator. The tolerance variations are corrected by trimming the center electrode tip. After the tip is trimmed, the ground electrode is bent to a final configuration to form a desired spark gap with the center electrode. The tolerance variations which require trimming the center electrode add to the cost of manufacturing the spark plug. Also, trimming the center electrode tip after the center electrode is assembled in the insulator and bending the ground electrode after the insulator assembly is mounted in the shell may place undesirable stresses on the insulator.